This invention relates to the field of photoluminescent (PL) analysis, in which a light source is used to excite a sample, and the photons emitted by the sample are passed through a monochromator to a detector which provides a measurement of intensity.
The present invention is particularly valuable as a means of determining impurity concentrations in single crystal silicon (Si), but it also has other potential uses, such as dopant measurement in gallium arsenide. Generally, the impurities would be those unintentionally incorporated; but in some cases intentionally doped impurities would be measured. Such impurity determinations are a very important means of determining the characteristics of electronic devices in integrated circuit chips.
The use of PL analysis for this purpose is discussed in an article by Tajima in Applied Physics Letters (American Institute of Physics), Volume 32, No. 11, June 1, 1978 (Page 719), and in an article by Tajima and Nomura in Japanese Journal of Applied Physics, Volume 20, No. 10, October, 1981, (Page L-697). As pointed out in these articles, the PL technique "can be successfully applied to the characterization of silicon crystals as a powerful means for analysis of shallow impurities. The PL method makes it possible to detect non-destructively a small amount of impurities in a small region of a specimen". The cited articles point out that the concentration of an impurity is proportional to the ratio of the intensities of the impurity and intrinsic signals.
The use of PL analysis of silicon chips was discussed by L. W. Shive, of the Monsanto Company, in October 1981, at a meeting of ASTM F1.06, The Electrical and Optical Measurements Committee. It was point out that, in his experiments, PL analysis was "designed to analyze single crystalline silicon only"; and that the method "assays silicon for Group II B and V B impurities, that is boron, phosphorus, arsenic, aluminum, and antimony". As summarized by Shive, PL analysis basically involves three steps: (1) "low temperature photoexcitation of the silicon sample", (2) "light emission from the sample--luminescence", and (3) "detection of the emitted light. The luminescence is a result of carrier recombination which takes place within the silicon sample".
In discussing the apparatus used for experimental purposes, Shive stated: "A laser is used to photoexcite the sample--which is immersed in liquid helium. The light emitted by the sample is resolved by a monochromator, detected by a photomultiplier tube, amplified, and recorded. The sample luminesces continuously and a spectrum of intensities as a function of wavelength is recorded."
The very promising concepts discussed above are subject to the problem of getting as much light as possible from the sample through the monochromator and onto the photomultiplier tube. To do this, one must collect as much of the light being emitted by the sample as possible, and fill the monochromator's entrance aperture and acceptance cone with this light. The problem at first seems straight forward. One collects the light from the sample with a colliminating lens of low f number, then focuses the light onto the monochromator's entrance slit with a lens which matches the monochromator's acceptance f number. Since monochromators typically have f numbers between f/3 and f/5, the focusing lens can be a single element. With this pair of lenses, the solid angle requirements are met.
In the PL analysis systems heretofore used, only about 2% of the available sample-emitted light collected by the adjacent lens passes the entrance slit in the monochromator. The round spot illuminated by the laser (source of incident light at the sample), when transferred to the monochromator entrance slit, is still round. The slit, however, is long and narrow, e.g., approximately 0.012 mm.times.6 mm, causing a severe mismatch. Attempts to distort the spot image, to make it better match the slit, run afoul of the solid angle consideration which dictated the initial pair of lenses. Essentially, anything gained by changing the spot shape is lost by angular mismatch. One possible solution of the problem is the use of fiber optic image transformers. However, their low packing density (30%) and high cost make them unattractive.